BS.1770, in one sentence
ITU-R BS.1770 is the international standard that specifies how to measure the perceived loudness of audio — it defines the frequency weighting and the gating rules that turn a waveform into a single LUFS number. Every streaming platform's loudness normalisation, and the EBU R128 broadcast standard, are built on BS.1770.
K-weighting: measuring loudness the way ears hear it
Human hearing isn't equally sensitive to all frequencies, so BS.1770 first passes the audio through a K-weighting filter with two stages: a high-shelf pre-filter that gently boosts high frequencies (roughly +4 dB above ~1.5 kHz, modelling the head-related boost), and a RLB high-pass that rolls off very low frequencies (around 38 Hz) because deep bass contributes little to perceived loudness. The result correlates with how loud a track actually sounds, not just its raw energy.
Gating: ignoring the silences
A song has quiet passages that would drag the average down, so BS.1770 applies two gates. The absolute gate discards blocks below -70 LUFS (near-silence). The relative gate then discards blocks more than 10 LU below the average of what remains. What's left is measured to produce the Integrated LUFS — the headline loudness figure platforms use.
How a LUFS reading is built
The pipeline is: 1) K-weight both channels; 2) split into 400 ms blocks with 75% overlap; 3) compute mean-square power per block; 4) apply the -70 LUFS absolute gate and the -10 LU relative gate; 5) average the surviving blocks and convert to LUFS. Loudness Range (LRA) is derived separately from the spread of 3-second short-term measurements.
BS.1770 vs EBU R128
BS.1770 defines the measurement method. EBU R128 is a broadcast recommendation built on BS.1770 that adds a target (-23 LUFS), a true-peak ceiling (-1 dBTP) and the LRA descriptor. Streaming platforms use their own targets (around -14 LUFS) but the same BS.1770 measurement underneath. So a BS.1770-accurate meter reads correctly for both broadcast and streaming.
Why an exact implementation matters
Many web meters approximate K-weighting with generic parametric-EQ (RBJ) biquads, which drift from the spec's exact filter — the deviation can reach several tenths of a LU in the presence region around 3 kHz. Quantara Music implements the exact BS.1770-4 K-weighting using the published De Man coefficients, recomputed for your file's native sample rate, so the reading matches the standard at both 44.1 kHz and 48 kHz. Accurate metering is the whole point: if the number is wrong, every loudness decision built on it is wrong.
Frequently asked questions
K-weighting is the frequency filter in BS.1770 that shapes audio to match human loudness perception before measurement: a high-frequency shelf boost plus a low-frequency roll-off. It's why LUFS tracks perceived loudness rather than raw energy.
BS.1770 is the ITU measurement method (K-weighting + gating). EBU R128 is a broadcast recommendation built on it, adding a -23 LUFS target, a -1 dBTP ceiling and Loudness Range. Streaming uses BS.1770 with different targets (~-14 LUFS).
Because many meters approximate the K-weighting filter or handle gating and sample rates differently. A spec-exact implementation (correct De Man coefficients per sample rate, proper -70/-10 gating) removes those discrepancies.
Yes. Quantara uses the exact BS.1770-4 K-weighting (De Man coefficients recomputed per sample rate) with correct absolute and relative gating, so integrated LUFS matches the standard at 44.1 kHz and 48 kHz.